AU734286B3

AU734286B3 - Method of continuous magnesium refining

Info

Publication number: AU734286B3
Application number: AU59519/00A
Authority: AU
Inventors: Ivan A. Barannik; Alexander M. Bashmakov; Andrey P. Gerb; Ellen L. Kaluzskaya
Original assignee: State Titanium Res And Design Institute
Current assignee: STATE TITANIUM RESEARCH AND DESIGN INSTITUTE
Priority date: 2000-09-20
Filing date: 2000-09-20
Publication date: 2001-06-07
Anticipated expiration: 2006-09-20

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A PETTY PATENT

ORIGINAL

Name of Applicant: Actual Inventor: Address of Service: Invention Title: STATE TITANIUM RESEARCH AND DESIGN INSTITUTE Ivan A. Barannik, Ellen L. Kaluzskaya, Alexander M.

Bashmakov and Andrey P. Gerb BALDWIN SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "METHOD OF CONTINUOUS MAGNESIUM REFINING" The following statement is a full description of this invention, including the best method of performing it known to us:- -la- METHOD OF CONTINUOUS MAGNESIUM REFINING Field of the invention This invention relates to the metallurgy of non-ferrous metals, and in particular to a method of manufacturing of refined magnesium.

Description of the Prior Art A method of continuous magnesium refining is known in the prior art (see Patent of Ukraine No 8612, dated of 30.09.96). This method consists of a step of refining magnesium by utilizing a salt melt of alkali, alkali-earth metals and magnesium and of an accompanying subsequent step of settling magnesium over the melt layer. In this method, correction of the salt melt composition occurs periodically. The salt melt contains the following components (percent by mass): MgC1 2 10.0; NaCl, 15.0; CaC1 2 BaC12, 5.5; and KC1 the remainder. It is desirable to maintain the differential between the density of the salt melt and that of magnesium at a level of at least 0.03 g/cm 3 To maintain the desired density differential, BaC12 is sued in the melt as a "weighting component." The inclusion of this 2 component in the salt melt provides separation between the metal and the salt melt, especially when the differential between the respective temperatures thereof does not exceed 10-15 0 C. During refining, as the temperature of the salt melt increases, the viscosity thereof decreases, and magnesium penetrates into the salt melt to settle in the slime. This results in increased magnesium losses during the refining process. In this prior art method, because the refining salt melt functions as the electrically conductive heat transfer medium, the above-mentioned increase in the temperature is inevitable. This is particularly apparent at a zone in which the electrodes are introduced into the working chamber. This zone is known for the local overheating of the salt melt.

Because of the increase in the salt melt fluidity, magnesium penetrates into spaces between the electrodes situated at the lower part of the furnace. The presence of the magnesium within these spaces results in a change in the electrical resistance of the conductive medium which, in turn, ultimately leads to disrupting of the operation of the electrical transformers serving the furnace.

In view of the shortage and toxicity of barium chloride, utilization of additional quantity of this compound as a "weighting component" is not advisable. This is because such use -3of barium chloride results in deterioration of local ecology and in the increased cost of refined magnesium.

Further, the use of the barium chloride in the refining process often prevents utilization of the spent melt as an additive in the production of fertilizers. Since 30-50 kg of waste is formed per ton of produced refined magnesium, storage of such large quantities is a major waste-disposal undertaking.

The use of calcium chloride as a "weighting component" is preferable in the product of the refined magnesium. However, when used as a "weighting component" the required quantity of calcium chloride is much greater than that of barium chloride.

When excess calcium chloride is used, the melt has increased viscosity and hydroscopicity. This increase inevitably causes an increase of the magnesium losses with slime. The following chemical reactions illustrate the losses of magnesium with the slime: CaCI 2

H

2 0 CaO 2HCl T CaO MgCI 2 MgO CaCI 2 Mg 2HCI MgCI 2

H

2 This dispersed magnesium oxide formed during these reactions entraps particles of magnesium metal, and as a result, deposits of magnesium are formed at the bottom of the apparatus Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Summary of the Invention One aspect of the invention provides a method of continuous refining of magnesium consisting of the following steps: treatment of crude magnesium with a slat -4melt of halogenides of alkali, alkali-earth metals and magnesium; settling of magnesium over the salt melt; and periodical correction of the salt melt composition, wherein components of the slat melt are taken in combination and proportion, so as to provide a temperature of its melting within the range between 450' and 650'C and to provide difference between the density of the slat melt and the density magnesium within the range between 0.03 and 0.2 g/cm 3 Preferably, the combination and proportion of the component of the salt melt utilized in the refining are as follows (percent by mass): MgC12 5.0-15.0 NaCl 15.0-50.0 CaC12 1.0-20.0 NaBr 0.1-10.0 CaF 2 0.5-2.0 KCI the remaining.

More preferably, the step of periodical correction of the slat melt is carried out by periodical introduction of powdered calcium fluoride and dehydrated calcium chloride into the melt, so that the introduction of these ingredients do not exceed one third of the mass of the melt.

Description of the Preferred Embodiment According to this invention, a method of continuous magnesium refining is described herein by which refined magnesium is settled over a layer of salt melt. This salt melt layer consists of the halogenides of alkali, of alkali-earth metals and magnesium. The composition of slat melt is periodically corrected. The components of the salt melt are taken in such combination and proportion so as to have a melting temperature in the range between 4500 and 650 0 C, and to maintain a density differential between the salt melt and that of the magnesium metal between 0.03 and 0.2 g/cm 3 To achieve these results, the components utilized in the salt melts are as follows: (percent by mass): MgCl 2 5.0-15.0; NaC1, 15.0-50.00; CaC12, 1.0-20.0; NaBr, 0.1-10.0; CaF 2 0.5-2.0; and KC1 the remainder. In the invention, the correction or adjustment of the salt melt 6 composition is accomplished by periodically introducing CaF 2 in the form of powder as well as dehydrated CaC1 2 and NaBr.

Furthermore, a portion of the salt melt not exceeding one third of its total mass or quantity is substituted by the freshly prepared melt.

In order to provide the required differential between the density of the salt melt and that of magnesium, calcium chloride and sodium bromide are utilized as "weighting components" of the salt melt. However, the required quantity of these "weighting components" which is necessary for this task, has to be greater than the quantity of barium chloride. The large quantity of calcium chloride (more than 20 percent by mass) in the melt results in the increase of hygroscopicity and viscosity of the melt. Such quantity of calcium chloride also leads to the losses of magnesium with slime.

When calcium chloride is introduced in the quantity not exceeding 1 percent by mass, the required differential between the density of the salt melt and that of magnesium metal has not been observed. In this instance the magnesium metal does not remain at the surface of the salt melt, but is partially submerged to the bottom of the device, so that the losses of the magnesium with the slime are increased. In addition, the fraction of the chloride-ion in the total mass of the refined 7 metal has increased from 0.005 to 0.010 percent by mass. This does not meet the quality requirements of primary magnesium.

When the calcium chloride content is at the lower limit, in order to reduce the hygroscopicity of the melt, sodium bromide is introduced into the salt melt. The major requirements of this process are as follows: the melting temperature of the salt melt is in the predetermined range between 450* and 650 0 C; and the difference between the density of the salt melt and that of magnesium is within the range of 0.03 and 0.2 g/cm 3 To satisfy such requirements, the desired level of the hygroscopicity of the salt melt is achieved by introducing of sodium bromide in a quantity of between 5 and 10 percent by mass.

This decreases the calcium chloride content in the total mass of the salt melt and, in turn, ultimately decreases the hygroscopicity and viscosity of the melt. As a result, the losses of magnesium into the slime are reduced. When sodium bromide is introduced into the melt in quantities not exceeding 0.1 percent by mass, in order to adjust the density to a predetermined level, it is necessary to maintain the calcium chloride content in the melt at its upper limit.

In view of the cost of calcium bromide, it is not economical to use this component in the quantities exceeding 10 percent by mass. Also, there is no indication that when during the refining process calcium bromide is used in quantities exceeding 8 percent, there is a further reduction of losses of magnesium in the slime.

Calcium fluoride is a destabilizing component utilized in the formation of magnesium emulsion in the melt. This component is introduced into the melt in quantities between 0.5 and percent by mass to counteract the deleterious effect of any dispersed magnesium oxide particles. The latter component is used as a stabilizer of magnesium emulsion. Thus, the presence of calcium fluoride facilitates purification and enhances agglomeration of liquid magnesium drops, thereby reducing the metal losses during the refining process.

It has been found that the deleterious effect of dispersed magnesium oxide is not counteracted when the content of calcium fluoride in the total mass of the salt melt is less than percent. When the content of calcium fluoride exceeds 2 percent of the total mass, further reduction of magnesium losses in the slime has not been observed. Because the solubility of calcium fluoride in the working salt melt at the temperature range of the step of refining is limited, the increase in calcium fluoride losses into the slime has been observed.

Concurrently, calcium fluoride saturates the salt melt with calcium chloride. As a result, the following reversible reaction takes place: CaF 2 MgCl2---CaCl 2 MgF 2 9 According to the method of the invention, the salt melt containing halogenides of alkali, alkali-earth metals and magnesium is introduced into the furnace. Upon the salt melt being heated to a temperature in the range of 7200-730' C (approx.), crude magnesium is introduced into the furnace. After reaching the temperature range of 700-7200 C, the crude magnesium is settled for approximately 0.3-0.5 hour. After settling, magnesium is poured onto a casting conveyer or, in the alternative transported to a vacuum ladle for titanium reduction.

The content of the following components in the salt melt composition is monitored a daily basis: calcium chloride, magnesium chloride, sodium chlorides, sodium bromide, calcium fluoride and magnesium oxide. Upon the salt melt gradually becoming contaminated with by-products of magnesium refining process, (such as: oxides, nitrides and iron), the need for periodical correction of salt melt composition arises. For this purpose, about one third of the melt is removed and magnesium chloride, sodium chloride, and potassium chloride are introduced into the furnace. Then, calcium fluoride and dehydrated calcium chloride and sodium bromide in the form of a powder are fed into the furnace.

10 Upon accumulation, the slime is removed from the furnace and periodically analyzed to determine the content of magnesium metal and magnesium oxide.

The data, characterizing the influence of the salt melt ingredients, is presented in Table i.

The supporting data is generated using a laboratory furnace of the retort type. The capacity of the retort accommodates about 30 kg of magnesium and about 40 kg of salt melt. The concentration of magnesium and sodium chlorides is maintained at the same level during all test runs. Similarly, concentration of magnesium oxide in the salt melts is monitored so as not to exceed 0.5 percent by mass.

According to the tabulated results (See Table the lowest content of magnesium in the slime corresponds to the following composition of elements: the fraction of total calcium chloride mass was about 10-20 percent, total calcium fluoride mass was between 0.5 and 1.6 percent and the total mass of sodium bromide was between 5 and 10 percent (tests 3, 4, and These tests indicate process optimization utilizing the method of the present invention.

Utilization of the invention reduces the loss of magnesium in the slime in the ratio 2:1 as compared to the prior art. In addition, with exception of the use of barium chloride, the ecological conditions of the process are improved. The invention 11 also reduces the labor expenses incurred in magnesium refining.

Furthermore, as the spent salt melt does not contain high toxic barium chloride it is utilizable for by-products.

The method of magnesium refining of the invention consists of the steps of the treatment of crude magnesium with salt melt of halogenides of alkali, alkali-earth metals and magnesium accompanied by the subsequent settling the metal over the salt melt. The combination and proportion of the components of the salt melt are as follows (percent by mass): MgCl 2 5.0-15.0; NaCl, 15.0-50.0; CaC1 2 1.0-20.0; NaBr, 0.1-10.0; CaF 2 and, Kcl the remaining portion. Maintaining of the above combination and proportion of the components of the salt melt resulted in maintaining the temperature of melting within the range of between 4500 and 6500 C; and also enables the invention to maintain the difference between the density of the melt and the density of metal within the range between 0.03 and 0.2g/cm 3 During the process, the salt melt composition is corrected by periodically introducing powdered calcium fluoride and dehydrated calcium chloride. The maintenance of the salt melt composition is monitored, so that the introduction of ingredients does not exceed one third of the volume of the melt.

12 TABLE 1 No. Fraction of the mass of salt melt components, Mg.content Test MgCl 2 NaCi CaC1 2 CaF 2 NaBr KCl BaCl 2 in slime %mass 1 10.0 50.0 1.0 2.2 0.1 36.7 2 5.0 20.0 5.0 2.0 1.2 66.8 3 10.0 30.0 10.0 1.6 5.0 43.4 -4.8 4 15.0 40.0 15.0 0.5 6.0 23.5 10.0 15.0 20.0 0.5 10.0 44.5 6 12.0 15.0 25.0 0.2 0.1 47.7 -7.3 7 10.0 15.0 28.0 0.2 0.1 46.7 -8.7 Prior Art 10.0 15.0 5 0 AA RRi1

Claims

1. A method of continuous refining of magnesium in a salt heated furnace consisting of the following steps: treatment of crude magnesium with a salt melt of halogenides of alkali, alkali-earth metals and magnesium; settling of magnesium over the salt melt; and periodical correction of the salt melt composition, wherein components of the salt melt are taken in combination and proportion so as to provide a temperature of its melting within a range between 4500 and 650'C and to provide difference between density of the salt melt and density of magnesium within the range between 0.03 and 0.2 3 g/cm

2. The method of claims 1, wherein combination and proportion of the components of the salt melt utilized in the refining of magnesium are as follows (percent by mass): MgC1 2

5.0-15.0 NaC1 15.0-50.0 CaC12 1.0-20.0 NaBr 0.1-10.0 CaF 2 0.5-2.0 KC1 the remaining. 3. The method of claim 1, wherein the steps of periodical correction of the salt melt is carried out by periodical introduction of powdered calcium fluoride and dehydrated -14- calcium chloride into the salt melt so that the introduction of the ingredients does not exceed one third of the mass of the melt. DATED this 10th Day of April 2001 STATE TITANIUM RESEARCH AND DESIGN INSTITUTE Attorney: PAUL G. HARRISON Fellow Institute of Patent and Trade Mark Attorneys of Australia of BALDWIN SHELSTON WATERS